Friday, November 13, 2015

Cheap light pollution filter

As soon as you say "filter" the public perception is that the images are photoshopped, or a little bit fake. I'm not one for heavily processing my images for other kinds of photography, and I'm more interested in being able to show what's up there in the night sky on the familiar terms that the general public is familiar with. There will always be limitations to this: "Is this what I would see?" doesn't exactly work in the world of nighttime photography in general.

Your eyes are more sensitive, but they can't accumulate light over time like a camera. So things can be much brighter in pictures.

Your eyes see vivid color in the day time, but the darker it gets the more your eyes rely on the "rods" of your retina, which are monochromatic; things appear a bit more blue than grey, but sensing reds, greens, and vivid blues is out.

You're looking through a telescope, which is a form of filtering on its own. Your viewing angle is cut down from maybe 170 degrees to 1-2 degrees. Your effective pupil size is also expanded from a few millimeters to the diameter of the telescope to gather more light.

But none of those things speak to real filters, which is what I'm adding to the system but going to try to keep the color "real" as much as possible, at least for now.

Astronomy filters can be very specific, and the cost of even the simpler ones is very high. I'm only a few miles outside of the city and live where the air is quite thick, which means street lamps add an orange glow to the sky and really get in the way of seeing what's up there. The goal is to take pictures of things outside of our atmosphere, not the atmosphere itself, right?

The orange comes from sodium, and the good news is that there is a cheap solution to this. "Red enhancing" or Didymium filters are made with neodymium. This happens to block that range of light without blocking much else. Amazon had the 52mm version for $23, which happens to be exactly what I need. I just had to remove it from the threaded lens mount so I could put it in to the telescope:

You can see by looking at the white cloth under the filter that it doesn't change the color very much.

I wedged it in to the T2-FX adapter and used some cardboard as a spacer. I will replace the cardboard with something better and less reflective soon, but this was more than enough to test with.